Additive manufacturing method for discharging interlocking continuous reinforcement

ABSTRACT

An additive manufacturing method is disclosed. The method may include directing into a print head a reinforcement having a continuous axial core and integral branches extending radially outward from the continuous axial core. The method may also include coating the reinforcement in a matrix, and softening a portion of a track of the coated reinforcement that was previously discharged from the print head. The method may further include discharging from the print head a track of the coated reinforcement adjacent the previously discharged track of the coated reinforcement, such that cross-bonding of the integral branches occurs between the discharging track of the coated reinforcement and the softened portion of the previously discharged track of the coated reinforcement.

TECHNICAL FIELD

The present disclosure relates generally to a manufacturing method and,more particularly, to an additive manufacturing method for discharginginterlocking continuous reinforcement.

BACKGROUND

Continuous fiber 3D printing (a.k.a., CF3D™) involves the use ofcontinuous fibers embedded within material discharging from a moveableprint head. A matrix is supplied to the print head and discharged (e.g.,extruded and/or pultruded) along with one or more continuous fibers alsopassing through the same head at the same time. The matrix can be atraditional thermoplastic, a powdered metal, a liquid resin (e.g., a UVcurable and/or two-part resin), or a combination of any of these andother known matrixes. Upon exiting the print head, a cure enhancer(e.g., a UV light, an ultrasonic emitter, a heat source, a catalystsupply, etc.) is activated to initiate and/or complete curing of thematrix. This curing occurs almost immediately, allowing for unsupportedstructures to be fabricated in free space. And when fibers, particularlycontinuous fibers, are embedded within the structure, a strength of thestructure may be multiplied beyond the matrix-dependent strength. Anexample of this technology is disclosed in U.S. Pat. No. 9,511,543 thatissued to Tyler on Dec. 6, 2016 (“the '543 patent”).

Although continuous fiber 3D printing provides for increased strength,compared to manufacturing processes that do not utilize continuous fiberreinforcement, inter-layer strength can be too low for someapplications. Specifically, when new material is discharged over the topof a layer of existing and already-cured material, the bond between thelayers of material may be low due to a lack of fibers extending betweenthe layers. And in some applications, this reduced inter-layer strengthmay be problematic.

The disclosed system and method are directed to addressing one or moreof the problems set forth above and/or other problems of the prior art.

SUMMARY

In one aspect, the present disclosure is directed to a method ofadditively manufacturing a composite structure. The method may includedirecting into a print head a reinforcement having a continuous axialcore and integral branches extending radially outward from thecontinuous axial core. The method may also include coating thereinforcement in a matrix, and softening a portion of a track of thecoated reinforcement that was previously discharged from the print head.The method may further include discharging from the print head a trackof the coated reinforcement adjacent the previously discharged track ofthe coated reinforcement, such that cross-bonding of the integralbranches occurs between the discharging track of the coatedreinforcement and the softened portion of the previously dischargedtrack of the coated reinforcement.

In another aspect, the present disclosure is directed to another methodof additively manufacturing a composite structure. This method mayinclude directing a continuous reinforcement into a print head, andwetting the continuous reinforcement with a reversible matrix. Themethod may further include exposing a portion of a track of the wettedcontinuous reinforcement that was previously discharged from the printhead and cured to a deactivation energy. The method may also includedischarging from the print head a track of the wetted continuousreinforcement adjacent the exposed portion of the previously dischargedtrack, and exposing at least the discharging track of the wettedcontinuous reinforcement to a cure energy to cause curing of thereversible resin.

In yet another aspect, the present disclosure is directed to a systemfor additively manufacturing a composite structure. The system mayinclude a support moveable in multiple dimensions, and a print headconnected to an end of the support. The print head may have a nozzle,and the system may further include a cure enhancer located at a trailingedge of the nozzle, and a deactivator located at a leading edge of thenozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrammatic illustrations of an exemplary disclosedmanufacturing system;

FIGS. 3, 7, and 8 are enlarged diagrammatic illustrations of exemplarydisclosed portions of the manufacturing systems of FIGS. 1 and 2; and

FIGS. 4, 5, and 6 are cross-sectional illustrations of an exemplaryfilament that may be used in conjunction with the manufacturing systemof FIGS. 1 and 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary system 10, which may be used tocontinuously manufacture a composite structure 12 having any desiredcross-sectional shape (e.g., circular, polygonal, etc.). System 10 mayinclude at least a support 14 and a head 16. Head 16 may have a body 18that is coupled to and moved by support 14, and a nozzle 20 located atan opposing discharge end of body 18. In the disclosed embodiment ofFIG. 1, support 14 is a robotic arm capable of moving head 16 inmultiple directions during fabrication of structure 12, such that aresulting longitudinal axis of structure 12 is three-dimensional. It iscontemplated, however, that support 14 could alternatively be anoverhead gantry or a hybrid gantry/arm also capable of moving head 16 inmultiple directions during fabrication of structure 12. Although support14 is shown as being capable of 6-axis movements, it is contemplatedthat any other type of support 14 capable of moving head 16 in the sameor in a different manner could also be utilized, if desired. In someembodiments, a drive may mechanically couple head 16 to support 14, andmay include components that cooperate to move and/or supply power ormaterials to head 16.

Body 18 may be configured to receive or otherwise contain a matrix. Thematrix may include any type of material (e.g., a liquid resin, such as azero volatile organic compound resin; a powdered metal; etc.) that iscurable. Exemplary resins include thermosets, single- or multi-partepoxy resins, polyester resins, cationic epoxies, acrylated epoxies,urethanes, esters, thermoplastics, photopolymers, polyepoxides, thiols,alkenes, thiol-enes, and more. In one embodiment, the matrix inside body18 may be pressurized, for example by an external device (e.g., anextruder or another type of pump—not shown) that is fluidly connected tohead 16 via a corresponding conduit (not shown). In another embodiment,however, the pressure may be generated completely inside of body 18 by asimilar type of device. In yet other embodiments, the matrix may begravity-fed through and/or mixed within body 18. In some instances, thematrix inside body 18 may need to be kept cool and/or dark to inhibitpremature curing; while in other instances, the matrix may need to bekept warm for the same reason. In either situation, body 18 may bespecially configured (e.g., insulated, chilled, and/or warmed) toprovide for these needs.

The matrix may be used to coat, encase, or otherwise surround any numberof continuous reinforcements (e.g., separate fibers, tows, rovings,and/or sheets of material) and, together with the reinforcements, makeup at least a portion (e.g., a wall) of composite structure 12. Thereinforcements may be stored within (e.g., on separate internalspools—not shown) or otherwise passed through body 18 (e.g., fed fromexternal spools 22—See FIG. 2). When multiple reinforcements aresimultaneously used, the reinforcements may be of the same type and havethe same diameter and cross-sectional shape (e.g., circular, square,flat, etc.), or of a different type with different diameters and/orcross-sectional shapes. The reinforcements may include, for example,carbon fibers, vegetable fibers, wood fibers, mineral fibers, glassfibers, metallic wires, optical tubes, etc. It should be noted that theterm “reinforcement” is meant to encompass both structural andnon-structural types of continuous materials that can be at leastpartially encased in the matrix discharging from nozzle 20.

The reinforcements may be exposed to (e.g., at least partially coatedwith) the matrix while the reinforcements are passing through body 18.The matrix, dry reinforcements, and/or reinforcements that are alreadyexposed to the matrix (a.k.a., wetted reinforcements) may be transportedinto body 18 in any manner apparent to one skilled in the art.

The matrix and reinforcement may be discharged from nozzle 20 as a trackof composite material via at least two different modes of operation. Ina first mode of operation, the matrix and reinforcement are extruded(e.g., pushed under pressure and/or mechanical force) from nozzle 20, ashead 16 is moved by support 14 to create the 3-dimensional shape ofstructure 12. In a second mode of operation, at least the reinforcementis pulled from nozzle 20, such that a tensile stress is created in thereinforcement during discharge. In this mode of operation, the matrixmay cling to the reinforcement and thereby also be pulled from nozzle 20along with the reinforcement, and/or the matrix may be discharged fromnozzle 20 under pressure along with the pulled reinforcement. In thesecond mode of operation, where the composite material is being pulledfrom nozzle 20, the resulting tension in the reinforcement may increasea strength of structure 12, while also allowing for a greater length ofunsupported material to have a straighter trajectory (i.e., the tensionmay act against the force of gravity to provide free-standing supportfor structure 12).

The reinforcement may be pulled from nozzle 20 as a result of head 16moving away from an anchor point 24. In particular, at the start ofstructure-formation, a length of matrix-impregnated reinforcement may bepulled and/or pushed from nozzle 20, deposited onto anchor point 24, andcured, such that the discharged material adheres to anchor point 24.Thereafter, head 16 may be moved away from anchor point 24, and therelative movement may cause the reinforcement to be pulled from nozzle20. It should be noted that the movement of reinforcement through body18 could be assisted (e.g., via one or more internal and/or externalfeed mechanisms—not shown), if desired. However, the discharge rate ofreinforcement from nozzle 20 may primarily be the result of relativemovement between head 16 and anchor point 24, such that tension iscreated and maintained within the reinforcement. It is contemplated thatanchor point 24 could be moved away from head 16 instead of or inaddition to head 16 being moved away from anchor point 24.

One or more cure enhancers (e.g., a UV light, an ultrasonic emitter, alaser, a heater, a catalyst dispenser, etc.) 26 may be mounted proximate(e.g., within, on, and/or trailing from) head 16 (e.g., at a base ofbody 18, inside of body 18, outside of body 18, or otherwise adjacentnozzle 20) and configured to enhance a cure rate and/or quality of thematrix as it is discharged from head 16. Cure enhancer 26 may becontrolled to selectively expose internal and/or external surfaces ofstructure 12 to energy (e.g., UV light, electromagnetic radiation,vibrations, heat, a chemical catalyst, hardener, or initiator, etc.)during the formation of structure 12. The energy may increase a rate ofchemical reaction occurring within the matrix, sinter the material,harden the material, or otherwise cause the material to cure as itdischarges from head 16.

A controller 28 may be provided and communicatively coupled with support14, head 16, and any number and type of cure enhancers 26. Controller 28may embody a single processor or multiple processors that include ameans for controlling an operation of system(s) 10 and/or 12. Controller28 may include one or more general- or special-purpose processors ormicroprocessors. Controller 28 may further include or be associated witha memory for storing data such as, for example, design limits,performance characteristics, operational instructions, matrixcharacteristics, reinforcement characteristics, characteristics ofstructure 12, and corresponding parameters of each component of system10. Various other known circuits may be associated with controller 28,including power supply circuitry, signal-conditioning circuitry,solenoid/motor driver circuitry, communication circuitry, and otherappropriate circuitry. Moreover, controller 28 may be capable ofcommunicating with other components of system 10 via wired and/orwireless transmission.

One or more maps may be stored in the memory of controller 28 and usedduring fabrication of structure 12. Each of these maps may include acollection of data in the form of lookup tables, graphs, and/orequations. In the disclosed embodiment, the maps are used by controller28 to determine desired characteristics of cure enhancers 26, theassociated matrix, and/or the associated reinforcements at differentlocations within structure 12. The characteristics may include, amongothers, a type, quantity, and/or configuration of reinforcement and/ormatrix to be discharged at a particular location within structure 12,and/or an amount, shape, and/or location of desired curing. Controller28 may then correlate operation of support 14 (e.g., the location and/ororientation of head 16) and/or the discharge of material from nozzle 20(a type of material, desired performance of the material, cross-linkingrequirements of the material, a discharge rate, etc.) with the operationof cure enhancers 26 such that structure 12 is produced in a desiredmanner.

In some applications, care must be taken to ensure that each of thefibers passing through head 16 are sufficiently coated with matrix(i.e., coated sufficient to ensure proper bonding and curing) prior todischarge from nozzle 20. As shown in FIG. 2, the fibers may be exposedto the matrix during travel through one or more chambers 30 that arelocated inside of body 18. The matrix may be supplied to chamber 30 inseveral different ways. For example, the matrix may be provided as agas-, a liquid-, or a powder-stream via a jet (not shown); as a bath viaa supply inlet (not shown); as a suspension via a pressurized conduit(not shown); or in another manner known in the art. In some embodiments,a teasing device (e.g., brushes, rollers, jets, etc.) and/or aregulating device (e.g., opposing rollers, a squeegee, a wiper, a brush,an air jet, etc.—not shown) may be disposed upstream and/or downstreamof chamber 30 to ensure adequate saturation and/or to remove excessmatrix component from the reinforcements prior to the coatedreinforcements entering nozzle 20. And upon the track of compositematerial exiting nozzle 20, curing may begin or speed up as a result ofexposure of the matrix to energy from cure enhancers 26.

In some embodiments, in addition to the matrix described above, anadditive may be mixed into the matrix. The additive may include, forexample, a filler, a hardener, a catalyst, and/or an initiator. Forinstance, a UV cure initiator could be mixed into the matrix, ifdesired. The UV cure initiator may be sufficient to raise a temperatureof the matrix coating the reinforcements to a minimum thresholdtemperature required for proper curing of the matrix.

As shown in FIGS. 3-6, the reinforcement (represented as R in FIGS. 3-6)being discharged through head 16 may have unique characteristics thatallow for increased inter-layer strength within structure 12. Forexample, the reinforcement R may include any number of individual fibersthat are oriented in a generally parallel relationship with each other.Each reinforcement R may have a continuous axial core C, and a pluralityof branches B that stem radially outward from the core C. The branches Band core C may be integral and/or fixedly connected (e.g., via welding,chemical bonding, etc.), such that, as the core C is discharged (e.g.,pulled) from nozzle 20, the branches B may be contemporaneouslydischarged from nozzle 20 due to their connections with the core C. Anynumber of branches B may stem from the same core C, with any axialand/or annular distribution. For example, each branch B may have adifferent axial stem location (see lower portion of FIG. 4) and/or adifferent annular stem location (See FIGS. 5 and 6). Alternatively,multiple branches B may have the same axial stem location (see upperportion of FIG. 4). The branches B may also have a repeated pattern ofannular stem locations (e.g., with 90° separation—see FIGS. 5 and 6) ora random pattern (not shown), as desired. Each branch B may have asingle extension (see FIG. 5) or multiple extensions (see lower-left ofFIG. 4), that extend orthogonally from the core C or at an inclined ordeclined angle. Further, the branches B may include only radiallyextensions (see FIG. 5) or both radial and vertical extensions (see FIG.6). Finally, the branches B may be completely linear or curvilinear.

Each branch B may have a natural or low-potential energy state, and adeformed or high-potential energy state. A branch B may extend afurthest radial distance from core C when in its natural state, and becompressed to a shorter radial distance when in its deformed state. Ingeneral, a branch B may be in its natural state outside of nozzle 20,and in its deformed state during passage through nozzle 20.

Once the reinforcement R has been coated with the matrix (represented byM in FIG. 3), a track of composite material may be discharged fromnozzle 20 that includes both the matrix M and the embedded reinforcementR. The track of material may have an outer diameter d, corresponding toa diameter of the coating of matrix M on the core C of the reinforcementR. The diameter d may be larger than an outer diameter of the core C,and about equal to a discharge diameter at a tip (e.g., a smallest innerdiameter) of nozzle 20. When the branches B are in their natural state,a diameter D of the reinforcement R (e.g., a radial distance between tipends of opposing branches B) may be larger than the diameter d. However,during passage of the reinforcement R through nozzle 20, an outerdiameter of the branches B may be reduced (e.g., compressed by innerwalls of nozzle 20) to be about the same as the diameter d. With thisconfiguration, after discharge of the reinforcement R from nozzle 20,the branches B may spring back to their natural state and extendradially out past the diameter d of the matrix coating on the core C.

The above-described extension of the branches B, radially outward pastthe diameter d of the matrix coating on the core C, may allow formechanical interlocking between adjacent tracks and/or overlappinglayers. For example, after a first layer of composite material isdischarged by nozzle 20, the tips of branches B in the first layer maybe exposed due to their radial-outward springing to their natural state.Thereafter, when a second layer of composite material is dischargedadjacent (e.g., over the top of) the first layer of composite material,the exposed tips of the branches B may penetrate the matrix coating ofthe overlapping core C. The matrix of the second layer may cure afterthis penetration, such that the branches B of the first layer areinternally bound within the matrix of the second layer.

In some embodiments, it may be beneficial for the branches B of thesecond layer discussed above to also be bound within the matrix of thefirst layer, such that cross-binding in multiple directions is achieved.This may be result in even greater interlayer strength. However, duringtypical fabrication of overlapping layers, the matrix of the first layeris generally cured and already hardened before the branches B of thesecond layer can penetrate the matrix of the first layer. For thisreason, a unique matrix and/or reaction deactivator (“deactivator”) 32may be implemented.

In the disclosed embodiment, the unique matrix is a “click” orreversible resin (e.g., a Triazolinedione, a covalent-adaptable network,a spatioselective reversible resin, etc.). A reversible resin is aphotopolymer having controllable molecularization. Specifically, when areversible resin is exposed to a first wavelength of light (e.g., about250 nm), the resin is cured during a phase change from a first state(e.g., a liquid state) to a second state (e.g., a solid state). And whenthe same resin is subsequently exposed to a second wavelength of light(e.g., about 300 nm), the resin reverses phase (e.g., only partially orcompletely) back to the first state. In the embodiment of FIG. 3, thefirst wavelength of light is provided by cure enhancer 26 during normaldischarge of a current track of composite material utilizing areversible resin as the matrix M surrounding the continuousreinforcement R. In this same embodiment, the second wavelength of lightis provided by way of deactivator 32.

As shown in FIG. 3, cure enhancer 26 and deactivator 32 may bestrategically located relative to a travel direction of head 16(represented by an arrow 34). For example, cure enhancer 26 may belocated to expose the green (e.g., uncured) composite materialdischarging from nozzle 20 at a currently layer (e.g., just behind thetip of nozzle 20), while deactivator 32 may be located to expose thealready cured composite material previously discharged by nozzle 20 atan adjacent layer (e.g., just in front of the tip of nozzle 20). Inother words, deactivator 32 may be located at a side of nozzle 20 thatis opposite cure enhancer 26. The exposure of the already curedcomposite material to the second wavelength of light at a location justin front of the tip of nozzle 20 may partially or completely reverse thecuring (e.g., soften or liquify the reversible resin) originallyimparted by the first wavelength of light, such that the exposed tips ofthe branches B of the currently discharging composite material maypenetrate the previously discharged material.

In an alternative embodiment shown in FIG. 7, the branches B are notexposed (i.e., do not extend through the matrix coating on the core C)immediately after discharge from nozzle 20. In particular, in thisembodiment, the branches B are not compressed during passage from nozzle20, such that the diameter D discussed above is about equal to or lessthan the diameter d. This arrangement can be used to simply increase thestrength of a single track of discharged composite material (e.g., byhaving reinforcements R in both axial and radial directions).Alternatively, this arrangement can be coupled with the use of acompactor 36 to provide mechanical cross-bonding between adjacent layerssimilar to that described above. For example, the tips of the branches Bmay be exposed only after compactor 36 has moved across and compactedthe discharging material. Compactor 36 may not only flatten out thematrix coating on the core C (e.g., reduce the diameter d to be lessthan the diameter D), but also press the discharging track of compositematerial onto the exposed branches B of the previously discharged layer.And in the same way described above, deactivator 32 may be selectivelyenergized (e.g., by controller 28—referring to FIG. 1) to soften orliquify the previously discharged layer at a leading edge of nozzle 20and compactor 36, such that compactor 36 may force the branches B of thedischarging track of material into the previously discharged layer.Compactor 36 may be, for example, a roller (shown), a shoe, or anotherdevice known in the art. It is contemplated that the composite materialmay be exposed to cure enhancing energy before (shown) and/or aftercompaction, as desired.

In some applications, deactivator 32 may be used to manipulate thepreviously discharged layer of composite material in additional ways.For example, deactivator 32 may be selectively energized to soften orliquify the previously discharged layer of composite material inpreparation for bending, twisting, warping, extraction and/orreplacement, etc. by compactor 36 or another auxiliary device (notshown). This may allow for complex shapes of structure 12 that werepreviously not possible.

In another alternative embodiment shown in FIG. 8, the branches B (oradditional branches B) may be separate from the core C and added to thecomposite material after discharge from nozzle 20. For example, whilethe discharging composite material is not yet fully cured, the branchesB may be pressed down into the composite material. The branches B may bepressed only part way through the composite material, such that thebranches extend from only one side of the composite material.Alternatively, the branches B may be pressed through the compositematerial and at least partway into a previously discharged and softenedor liquified layer of composite material. In the embodiment of FIG. 8, amodified compactor 38 is used to insert the branches B into thecomposite material. It is contemplated, however, that another device(e.g., a dedicated branch-insertion device—not shown) couldalternatively or additionally perform this function, if desired.

Industrial Applicability

The disclosed system may be used to continuously manufacture compositestructures having any desired cross-sectional shape, length, density,and/or strength. The composite structures may include any number ofdifferent reinforcements of the same or different types, diameters,shapes, configurations, and consists, and/or any number of differentmatrixes. Operation of system 10 will now be described in detail.

At a start of a manufacturing event, information regarding a desiredstructure 12 may be loaded into system 10 (e.g., into controller 28 thatis responsible for regulating operation of support 14, cure enhancer(s)26, jet(s) 30, regulating device(s) 36, fiber-teasing mechanism(s) 38,and/or any other associated components). This information may include,among other things, a size (e.g., diameter, wall thickness, length,etc.), a contour (e.g., a trajectory), surface features (e.g., ridgesize, location, thickness, length; flange size, location, thickness,length; etc.), connection geometry (e.g., locations and sizes ofcouplings, tees, splices, etc.), location-specific matrix stipulations,location-specific reinforcement stipulations, desired cure rates, curelocations, cure shapes, cure amounts, etc. It should be noted that thisinformation may alternatively or additionally be loaded into system 10at different times and/or continuously during the manufacturing event,if desired.

Based on the component information, a specific nozzle 20, cure enhancerconfiguration, and/or deactivator configuration may be connected to head16 (e.g., to the discharge end of body 18), and one or more different(e.g., different sizes, shapes, and/or types of) reinforcements and/ormatrixes may be selectively installed within system 10 and/orcontinuously supplied into nozzle 20. The corresponding reinforcements(e.g., prepreg or dry fibers, tows, ribbons, or sheets) may be passedthrough one or more fiber-teasing mechanisms (e.g., between the bristlesof adjacent brushes, and/or over or around protrusions, etc.—not shown)and nozzle 20, and thereafter connected to a pulling machine (not shown)and/or to a mounting fixture (e.g., to anchor point 24). Installation ofthe matrix may include filling chamber 30 with a reversible resin and/orcoupling of an extruder (not shown) to head 16.

Head 16 may be moved by support 14 under the regulation of controller 28to cause matrix-coated reinforcements to be placed against or on acorresponding anchor point 24. Cure enhancers 26 may then be selectivelyactivated (e.g., turned on at the first wavelength by controller 28) tocause hardening of the reversible resin surrounding the reinforcements,thereby bonding the reinforcements to anchor point 24.

The component information may then be used to control operation ofsystem 10. For example, the reinforcements may be pulled through thefiber-teasing mechanism; separated and/or flattened; submerged withinthe reversible resin, wrung out by any associated regulating device (notshown); and then discharged from nozzle 20. Controller 28 selectivelycause support 14 to move head 16 in a desired manner at this time, suchthat an axis of the resulting structure 12 follows a desired trajectory(e.g., a free-space, unsupported, 3-D trajectory). In addition, cureenhancers 26 may be selectively activated by controller 28 at the firstwavelength during material discharge to initiate, speed up, or completehardening of the reversible resin.

When tracks of composite material are to be discharged adjacent eachother and/or in an overlapping manner, controller 28 may selectivelyactivate deactivator 32 to expose the first track of material to thesecond wavelength of light. As described above, this exposure may softenor liquify the reversible resin in the first track of material, allowingpenetration of branches B from the discharging track of material. Inaddition, compactor 36 and/or 38 may be utilized to push the branches Bof adjacent tracks of material into the corresponding reversible resincoatings and thereby mechanically cross-bond the tracks. Once structure12 has grown to a desired length, structure 12 may be disconnected(e.g., severed) from head 16 in any desired manner

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed systems andhead. Other embodiments will be apparent to those skilled in the artfrom consideration of the specification and practice of the disclosedsystems and heads. It is intended that the specification and examples beconsidered as exemplary only, with a true scope being indicated by thefollowing claims and their equivalents.

What is claimed is:
 1. A method of additively manufacturing a compositestructure, comprising: directing into a print head a reinforcementhaving a continuous axial core and integral branches extending radiallyoutward from the continuous axial core; coating the reinforcement in amatrix; softening a portion of a track of the coated reinforcement thatwas previously discharged from the print head; and discharging from theprint head a track of the coated reinforcement adjacent the previouslydischarged track of the coated reinforcement such that cross-bonding ofthe integral branches occurs between the discharging track of the coatedreinforcement and the softened portion of the previously dischargedtrack of the coated reinforcement, wherein: exposing at least thedischarging track of the coated reinforcement to the cure energyincludes exposing at least the discharging track of the coatedreinforcement to light having a first wavelength; and softening theportion of the previously discharged track of the coated reinforcementincludes exposing the portion of the previously discharged track of thecoated reinforcement to light having a second wavelength different fromthe first wavelength.
 2. The method of claim 1, further includingexposing at least the discharging track of the coated reinforcement to acure energy to cause curing of the matrix.
 3. The method of claim 2,wherein exposing at least the discharging track of the coatedreinforcement to the cure energy includes exposing the discharging trackof the coated reinforcement and the softened portion of the previouslydischarged track of the coated reinforcement.
 4. The method of claim 3,further including manipulating the softened portion of the track ofcoated reinforcement prior to exposing the softened portion of the trackof coated reinforcement to the cure energy.
 5. The method of claim 1,wherein: the first wavelength is about 250 nm; and the second wavelengthis about 300 nm.
 6. The method of claim 1, wherein discharging the trackof the coated reinforcement includes discharging the track of the coatedreinforcement through a nozzle having an inner diameter less than anouter diameter extending between opposing tips of the integral brancheswhen the integral branches are in a natural state.
 7. The method ofclaim 6, wherein the integral branches are compressed during passagethrough the nozzle and return to a natural state after discharging fromthe nozzle.
 8. The method of claim 1, wherein coating the reinforcementin the matrix includes coating the reinforcement at a location inside ofthe print head.
 9. The method of claim 1, further including moving theprint head in multiple locations during discharging of the track of thecoated reinforcement.
 10. A method of additively manufacturing acomposite structure, comprising: directing into a print head areinforcement having a continuous axial core and integral branchesextending radially outward from the continuous axial core; coating thereinforcement in a matrix; softening a portion of a track of the coatedreinforcement that was previously discharged from the print head; anddischarging from the print head a track of the coated reinforcementadjacent the previously discharged track of the coated reinforcementsuch that cross-bonding of the integral branches occurs between thedischarging track of the coated reinforcement and the softened portionof the previously discharged track of the coated reinforcement, whereinthe matrix is one of a Triazolinedione, a covalent-adaptable network,and a spatioselective resin.
 11. The method of claim 1, furtherincluding compacting the discharging track of the coated reinforcementuntil the integral branches are exposed or the integral branches arepressed into the previously discharged track of the coatedreinforcement.
 12. The method of claim 1, further including pressingadditional branches into at least one of the track of the coatedreinforcement being discharged and the previously discharged track ofthe coated reinforcement.
 13. A method of additively manufacturing acomposite structure, comprising: directing a continuous reinforcementinto a print head; wetting the continuous reinforcement with areversible resin; exposing a portion of a track of the wetted continuousreinforcement that was previously discharged from the print head andcured to a deactivation energy to at least partially reverse a curingreaction of the portion of the track of the wetted continuousreinforcement that was previously discharged from the print head;discharging from the print head a track of the wetted continuousreinforcement adjacent the exposed portion of the previously dischargedtrack; and exposing at least the discharging track of the wettedcontinuous reinforcement to a cure energy to cause curing of thereversible resin.
 14. The method of claim 13, further including forcinga portion of the wetted continuous reinforcement discharging from theprint head into the exposed portion of the previously discharged track.15. The method of claim 14, wherein exposing at least the dischargingtrack of the wetted continuous reinforcement to the cure energy includesexposing at least the discharging track of the wetted continuousreinforcement to the cure energy only after the portion of the wettedcontinuous reinforcement has been forced into the exposed portion of thepreviously discharged track.
 16. The method of claim 15, wherein theportion of the wetted continuous reinforcement forced into the exposedportion of the previously discharged track extends from the reversibleresin only after discharge from the print head.
 17. A method ofadditively manufacturing a composite structure, comprising: wetting acontinuous reinforcement having an outer diameter with a liquid resinwhile inside of a print head; discharging the wetted continuousreinforcement through a nozzle of the print head having an innerdiameter less than the outer diameter of the continuous reinforcement;and following discharging of the wetted continuous reinforcement, curingthe liquid resin after expansion of the continuous reinforcement. 18.The method of claim 17, wherein: discharging the wetted continuousreinforcement includes discharging the wetted continuous reinforcementadjacent a previously discharged track of wetted continuousreinforcement; and during expansion of the continuous reinforcement, aportion of the wetted continuous reinforcement in the previouslydischarged track penetrates a track of the discharging wetted continuousreinforcement.